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Gallois, R. W. 2001. Geoscience in south-west England, Vol. 10, 243-246
1
FIELD EXCURSION TO EXAMINE THE GEOLOGY AND COASTAL
LANDFORMS OF THE CHARMOUTH TO LYME REGIS AREA, 3RD
JANUARY, 2001.
R. W. GALLOIS
Gallois R. W. Field excursion to examine the geology and coastal landforms of the
Charmouth to Lyme Regis area, 3rd January, 2001. Geoscience in south-west England,
10, 243-246.
INTRODUCTION
The aim of the field excursion was to demonstrate the more important geological
features that have controlled the development of the landforms in the coastal zone
between Charmouth and Lyme Regis, and to illustrate some of the results of recent
geological studies that have been designed to reach an understanding of the processes
involved. The visit was, in part, complementary to the description of the work carried out
to try to provide the data required to design remedial works that will save Lyme Regis
from the twin threats of landslip and marine erosion (Gallois and Davis, 2001).
When the fieldtrip was first planned in Autumn 2000, the intention was to traverse the
beach from Charmouth to Seven Rock Point, west of Lyme Regis, to examine the
stratigraphy and structure of the Jurassic rocks exposed in the cliffs, foreshore and Black
Ven, and the landslips and marine-erosion features adjacent to Lyme Regis. A number of
unusual events combined immediately prior to the visit to make this impracticable. First,
the unusually high rainfall in the early winter had reactivated many of the landslips along
the coast. A major landslip at Cain's Folly [SY 376 928] one week before the visit led the
police to close the beach and coastal path for several miles eastwards from Charmouth,
and to advise extreme caution on the beaches and landslipped areas west of Charmouth.
Second, south-westerly gales in December had transported much of the beach shingle
offshore with the result that beach levels were up to 3 m lower than normal, albeit with
better geological exposure in the intertidal zone. Third, strong winds on the day before
the visit had produced a heavy swell that carried the sea to the foot of cliffs that would
Gallois, R. W. 2001. Geoscience in south-west England, Vol. 10, 243-246
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usually be accessible. The field party therefore spent the morning at Charmouth and then
travelled by car to Lyme Regis for lunch and the afternoon.
CHARMOUTH AND BLACK VEN
The group followed the coastal path from Sea Lane car park [SY 365 931] at
Charmouth, in places skirting around the edge of recent landslip activity, to a National
Trust viewpoint [SY 357 934] on the eastern edge of the Tertiary plateau, adjacent to the
Roman Charmouth-Lyme Regis road. This locality provides splendid views eastwards
along the Dorset coastline and northwards inland to the Vale of Marshwood and beyond.
The westerly overstep of the Gault and Upper Greensand across the easterly dipping
Lower and Middle Jurassic rocks, the Tertiary planation surface, and extensive active
landslipping are clearly displayed in the cliffs below Stonebarrow Hill and Golden Cap.
Inland, the topographical expression of the same geology can be seen in the dissected
Tertiary plateau, the Cretaceous escarpments and the undulose landslipped hillsides that
run northwards from the coast.
To the west, the same viewpoint overlooks the great amphitheatre of the Black Ven-
Spittles landslip. The subsidiary failure mechanisms within the landslip were well
displayed at the time of the visit. A steady stream of collapsed masses of weathered sand
and chert from the Upper Greensand Formation (Whitecliff Chert Member) fell from the
backface of the landslip, and intermittent streams of sand from the lower part of the
Upper Greensand (Foxmould Member) flowed over the terrace-like escarpments that are
capped by thin (mostly ‹ 0.5 m thick) limestone beds within the Shales with Beef, Black
Ven Marl and Belemnite Marl members of the Jurassic Charmouth Mudstone Formation
(Figure 1). The principal failure mechanism, mass movement along shear surfaces in the
Gault Formation beneath the Upper Greensand, has rarely been observed because of the
almost permanent complete cover of collapsed Upper Greensand debris.
The Black Ven-Spittles landslip has been described as the largest active coastal landslip
in Europe (Brunsden, 1969). The area that is currently active consists of an upper bench
of about 0.2 km² of collapsed Upper Greensand that overlies a thin (1 to 2 m thick) bed of
Gault. Below this and extending to the sea, about 0.4 km² of mudflows and collapse-
debris derived from the Charmouth Mudstone and Upper Greensand rest on a series of
Gallois, R. W. 2001. Geoscience in south-west England, Vol. 10, 243-246
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benches between steep faces of Charmouth Mudstone (e.g. Conway, 1974). Both parts of
the currently active area are within a larger landslip complex that was probably formed in
the late Pleistocene in a periglacial climate when it was subjected to successive periods of
freezing and thawing of a deep permafrost layer. The Holocene rise in sea level has
caused active erosion of the toe of the 'fossil' landslip, which has become progressively
reactivated as erosion has proceeded. There is historical evidence to show that the rate of
reactivation has increased in the past 200 years, and it has been suggested that this may in
part be the result of increased marine erosion due to sea defence works and quarrying on
the foreshore at Lyme Regis.
The party walked westwards, close to the backface of the landslip, to a point on the
golf course at Timber Hill [SY 350 934] near the western limit of the active face, taking
care to stay on the landward side of a long line of tension cracks that presaged further
retreat of the backface. Westwards from this point, the Upper Greensand in the backface
was at that time vegetated by scrub and trees that became thicker and more mature in a
westerly direction. Six weeks after the visit, on February 14th 2001, a failure occurred for
a distance of c. 200 m along the tension cracks that had been observed on the visit, and
Figure 1. Simplified geological section through the central part of the Black Ven landslip showing the relationship of the topography to thin limestone beds within the Charmouth Mudstone Formation.
Gault hidden by mounds of collapsed Upper Greensand
coastalfootpath
150
160
140
120
100
80
60
40
20
00
Blue Liasexposed incliffs and onforeshore
Black Ven Marl
Whitecliff Chertdecalcified in part
Grey Ledgecaps feature
Belemnite Stone caps feature
Lower Cement Bed caps feature
Birchi Tabular caps feature
Stellare Nodules cap feature
Upper Cement Bed caps feature
Blue Lias
Shales with Beef
Belemnite Marl
NorthSouth
Foxmould
Gault
mudflows and patches of collapsed sand and mudstone omitted for clarity Vertical exaggeration: x1.25
500 400 300 200 100 metres
metres
Gallois, R. W. 2001. Geoscience in south-west England, Vol. 10, 243-246
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new tension cracks developed for another 250 m to the west. This was the longest single
reactivation of the Upper Greensand backface yet recorded, and it may prove to be the
most significant because it could regenerate a large area in the western part of the 'fossil'
landslip complex, adjacent to the Lyme Regis urban area (Figure 2).
Figure 1. .
Sketch diagrams of the western part of the Black Ven-Spittles 'fossil' landslip showing the reactivation between 1955 and 1996. Based on oblique air photographs; uncorrected for parallax
groynes
Church Cliffs
modern Lyme-Ch
armouth Road
modern Lyme-Ch
armouth Road
Upper Greensand Escarpment
1955
Stellare Nodules
Upper Cement Bed
Birchi Tabular
Belemnite Stone
Spittles Limestone
Grey Ledge
groynes
Church Cliffs
Cliffs/backfaces of Lias Mobile landslip debris
1996
'new' Charmouth Road
'new' Charmouth Road
Upper Greensand Escarpment
Upper Cement Bed
Lyme RegisUrban Area
Lyme RegisUrban Area
NENW
SW SE
SE
NENW
SW
('old' Charmouth Road)
('old' Charmouth Road)
Spittles Lane
Spittles Lane
Timber Hill
Timber Hill
probable inland limit of 'fossil' landslip
probable inland limit of 'fossil' landslip
cliffs of Blue Lias
cliffs of Blue Lias
western limit of activebackface at 14/2/2001
western limit of activebackface at time of visit
1000m
500m
900m
approximatearea of figure
1000m
500m
900m
approximatearea of figure
Belemnite Stone
Stellare Nodules
Birchi Tabular
Spittles Limestone
Grey Ledge
Gallois, R. W. 2001. Geoscience in south-west England, Vol. 10, 243-246
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The traverse of the backface of the landslip also afforded good views of what remains
of the 'new' Lyme Regis to Charmouth road. This ran along the foot of the then wooded
and stable Upper Greensand backface, presumably to avoid the extra climb up to the
Tertiary plateau. The 'new' road was opened in 1825 as a replacement for the 'old' road
which had run eastwards through the lower part of the landslip from what is now Spittles
Lane [SY 344 927] (Figure 2). The 'new' road was damaged by landslipping within three
years of its opening and, after many repairs, was eventually replaced by the modern road,
which runs inland to avoid the landslip, in 1880 (Woodward and Ussher, 1911, p. 80).
The 'new' road was finally closed to traffic in 1924. All that remains of it are a short
section within the Devil's Bellows road cutting [SY 357 933] at the eastern end and about
300 m at the western end [SY 346 932], much of which consists of a tarmac switchback
cut by deep fissures. In contrast, the Roman Road was not breached until 1995 when a
particularly large section of Upper Greensand backface collapsed after an unusually wet
autumn. Being on the Tertiary plateau, it could have been moved inland at little cost
if that had been required.
LYME REGIS
After lunch at the Cobb Arms, Lyme Regis, the party walked through sheets of sea
spray to the point on the outer wall of The Cobb [SY 3402 9142] made famous by Meryl
Streep in The French Lieutenant's Woman. There, the geographical conditions that made
the site so favourable for a harbour were described with the help of seismic-reflection
profiles commissioned by West Dorset District Council. These show that the original
outer wall of what is now the harbour complex was built on a limestone ledge (the Birchi
Tabular Stone Band of Lang, 1914) that provided protection from the prevailing south-
westerly winds. On the lee side of this ledge, the soft Shales-with-Beef give rise to a
sand-floored channel that gave access to the sheltered area behind the wall at all states of
the tide.
Early drawings and accounts show that the Cobb wall was not connected to the shore
until the 18th century. A low causeway was built to link it to the land when a new wall
was constructed in 1756 (Fowles, 1991). With time, this wall and the causeway were
Gallois, R. W. 2001. Geoscience in south-west England, Vol. 10, 243-246
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increased in height to produce the present-day wall, the landward end of which was last
raised in 1939 to provide year-round protection for an air-sea rescue facility.
Figure 2. .Changes in the intertidal area adjacent to The Cobb over the last 200 years
TheCobb
Cobb wall
River Lim
River Lim
River Lim
Broad Ledge
Church
Church
Church
HWMMT
HWMMT
HWMMT
HWMMT
LWMMT
LWMMT
LWMMT
LWMMT
LWMMT
LWMMT
LWMMT
LWMMT
LWMMT
Murdoch MacKenzie 1797
Admiralty chart 1854
Air Photograph 1996
Gravel (shingle) beach
Rock ledges, boulder strewn in part
Rock armour/Rip rap omitted for clarity
Sand beach
KEY
Degraded cliff of Shales-with-Beef
Landslip in Shales-with-beef
LWMMT
HWMMT High water mark median tides
Low water mark median tides
landscaped and built-up area
TheCobb
cross wall
Gallois, R. W. 2001. Geoscience in south-west England, Vol. 10, 243-246
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These changes to the Cobb wall have interfered with the natural west-to-east longshore
drift of beach shingle. This has resulted in a steady accumulation of shingle to the west of
the wall and the consequential loss of shingle from the town beach (Figure 3). One of the
earliest photographs of Lyme, (c. 1850, published by Fowles, 1990, Plate 11) shows a
short length of Shales-with-Beef cliff below what is now Lister Gardens. The removal of
the protective belt of shingle from the foot of this cliff probably encouraged its collapse
in the late 19th century, with the result that landslip movements have continued in that
area to the present day.
As the tide receded, the party walked westwards along the intertidal rock platform to
the eastern end of Pinhay Bay. The alternating limestones and shales of the Blue Lias
Formation are continuously exposed in the cliffs and on the foreshore between
Devonshire Head [SY 332 914] and Seven Rock Point [SY 328 909]. The limestone
ledges contain what Arkell (1933, p. 121) described as "probably the greatest profusion of
large ammonites to be seen anywhere in the British Isles". Many of the limestone bands
were worked for cement manufacture up to the time of the First World War. The stone
was transported to a cement works [SY 335 916] at Monmouth Beach via a narrow-gauge
railway than ran along the intertidal area. Short stretches of this were observed below
Devonshire Head.
ACKNOWLEDGEMENTS
The author is grateful to Geoff Davis and Geoff Kellaway for helpful discussion, and to
Dick Edwards for continuing advice and encouragement. Published by permission of the
Director, British Geological Survey (NERC).
REFERENCES
ARKELL, W. J. 1933. The Jurassic System in Great Britain. Clarendon Press, Oxford.
BRUNSDEN, D. 1969. The moving cliffs of Black Ven. Geographical Magazine, 41,
372-390.
CONWAY, B W. 1974. The Black Ven landslip Charmouth, Dorset. Report of the Institute of
Geological Sciences, No. 74/3.
FOWLES, J. 1990. Lyme Regis Camera. Dovecote Press, Wimborne.
Gallois, R. W. 2001. Geoscience in south-west England, Vol. 10, 243-246
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FOWLES, J. 1991. A short history of Lyme Regis. Dovecote Press, Wimborne.
GALLOIS, R. W. and DAVIS, G. M., 2001. Saving Lyme Regis from the sea: recent
geological investigations at Lyme Regis, Dorset. Geoscience in south-west
England, 10, 000-000.
LANG, W. D. 1914. The geology of the Charmouth cliffs, beach and foreshore. Proceedings
of the Geologists' Association, 25, 293-360.
WOODWARD, H. B. and USSHER, W. A. E. 1911. Geology of the Country near Sidmouth
and Lyme Regis. Memoir of the Geological Survey of Great Britain. Her Majesty’s
Stationery Office, London.